Method for blackening metals and novel compositions therefor



United States Patent 3,314,812 METHOD FOR BLACKENING METALS AND NOVEL COMPOSITIONS THEREFOR Abraham J. Mitchell, Stratford, Conn., assignor to The Mitchell-Bradfor l Chemical Co., Milford, Conn., a corporation of Connecticut No Drawing. Filed Jan. 17, 1964, Ser. No. 338,315 14 Claims. (Cl. 106-286) This invention relates to the method of blackening stainless steels while retaining to a high degree the corrosion-resistance thereof, and to the novel compositions for effecting the method.

There are a great number of conventional blackening compositions useful for the surface blackening of ferrous metals. Such conventional compositions generally com prise 50 to 90 parts by weight of an alkali metal hydroxide, 0.1 to 50 parts by weight of an alkali metal nitrate and 0.1 to 50 parts by weight of an alkali metal nitrite. While such compositions are quite effective for the blackening of most ferrous metals, they do not blacken stainless steel.

There are compositions known for the blackening of stainless steel but such compositions are diflicult to use in that the ingredients are not compatible but rather are coreactive so that such compositions must be mixed just prior to use and cannot be sold in premixed form. Such compositions also have a short useful life since the effective ingredients of such compositions are soon used up by interraction.

Stainless steel has a natural resistance to corrosion which is enhanced by passivating the stainless steel surface with a solution of nitric acid. It appears that the nitric acid oxidizes to a slight extent the metals such as chromium at the steel surface to form a very thin protective oxide film thereover. In order to blacken a stainless steel surface it is necessary to pickle the metal surface with an etching'solution such as one containing hydrochloric acid. This pickling step destroys the corrosion-resistance of the stainless steel, apparently by dissolving the protective oxide film, so that the steel is then in a condition very susceptible to rusting and other corrosion. Known stainless steel blackening compositions do not restore the corrosion-resistance lost in the pickling process, so that conventionally blackened stainless steel is very susceptible to corrosion within a relatively short time.

Finally, conventional blackening compositions olfer poor rinsability and thus create serious dragout problems since a large amount of the aqueous blackening composition tends to remain adhered to the backened stainless steel pieces upon their removal from the blackening bath. An excess of the aqueous composition adhered to the blackened pieces offers rinsing problems and rapid contamination of the rinsing bath, but more importantly results in a shortening of the effective life of the aqueous blackening composition due to the continuous depletion thereof in the form of unnecessary dragout.

It is an object of the present invention to modify conventional ferrous blackening compositions to render them completely effective for the blackening of stainless steel.

It is another object of this invention to provide stainless steel blackening compositions which form black surface deposits on stainless steel while restoring to a great extent the natural corrosion-resistance of the steel lost in the pickling process.

It is yet another object of the invention provide a stainless steel blackening composition which offers the advantages of improved rins-ability of the pieces being treated and reduced dragout of the solution by the pieces being treated.

It is an advantage of the present invention that the ingredients of the present compositions are completely compatible so that the compositions may be sold in premixed form, either wet or dry, and have a relatively long useful life.

It is another advantage of this invention to provide a stainless steel blackening composition which forms little or no smut on the stainless steel surface or smut-contamination of the composition bath.

These and other objects and advantages are accomplished by the present invention as will be clear to those skilled in the art in the light of the present disclosure.

The present invention resides in the discovery that conventional ferrous blackening compositions may be modified by the replacement of alkali metal nitrite with two or more compounds to produce compositions which are completely effective for the blackening of stainless steel, and which provide a black deposit having corrosion-resisting properties, and which 'are substantially completely free from smut-producing tendencies.

The additives of the present invention consist of a water-soluble sulfur-bearing compound together with a water-soluble molybdate salt and/or a water-soluble dichromate salt.

While alkali metal thiosulfates are the preferred Watersoluble sulfur-bearing compounds for use according to the present invention, a large number of other water-soluble sulfur-bearing compounds may be used in place thereof, including the alkali metal and ammonium salts of sulfides, thiocyanates and thiosulfates, as well as thioureas, Z-mercaptobenzothiazole. benzothiazyl disulfide, 2-rnercaptoimidazoline and the like. Such compounds may be used in amounts ranging from 0.1 up to parts by Weight based upon the total weight of the blackening composition.

The water-soluble molybdate salts such as sodium, potassium and ammonium molybdates and the like have been found e 'ective in amounts ranging from 0.0001 part by weight up to 20 or more parts by Weight based upon the total weight of the blackening composition.

The water-soluble dichromate salts such as sodium or potassium dichromate or the like have been found effective in amounts ranging from 0.0001 part by weight up to about 2 or more parts by Weight based upon the total weight of the blackening composition.

I have discovered that the blackening bath may be provided with improved properties with respect to rinsability and dragout by the addition of a Wetting agent which has a resistance to high alkalinity and oxidation at high temperatures between 220 F. and 310 F. Preferred for this factants PC-95, FC 98, FC-128, FC-134 and FX161. Such wetting agents should be added to the aqueous blackening composition in an amount ranging from about 0.0001 to 15 parts by weight based upon the total Weight of the composition.

It is not completely clear why the addition of molybdate and/or dichromate together with the sulfur-bearing compound to a conventional ferrous blackening composition in place of alkali metal nitrite renders the same effective for the blackening of stainless steel. The best plausible explanation appears to be that these enter into reaction with the chromium, nickel and/or iron of the stainless steel or otherwise affect these metals so that the inertness of the steel to blackening is overcome and a black coating is permitted to be formed on the stainless steel surface, said coating apparently being made up of black metal sulfides, black molybdic oxide and, possibly, black iron oxide.

It is not completely clear why the presence of alkali metal nitrite renders the present stainless steel blackening compositions inoperative but such result is apparently due to an interreaction between the nitrite and the chromium and molybdenum additives.

According to the preferred embodiment of the invention the blackening composition includes a mixture of the molybdate and the dichromate whereby exceptional results are accomplished with respect to the deposit of a uniform, smooth, dense black coating and the formation of blackened stainless steel having excellent corrosionresistance.

In carrying out the present invention, the stainless steel is prepared by degreasing the metal to remove oil, grease and dirt and then pickling the metal in a 50% hydrochloric acid solution followed by rinsing in cold water. Such treatment is conventional in the art for the preparation of metal surfaces to be plated.

The blackening step is accomplished by immersing the prepared stainless steel in an aqueous bath of any of the compositions of the following examples heated to a boiling temperatuer of 220 F. to 310 F., the immersion being continued for a sufficient period of time for the steel to take on a good black color, generally from about 1 to 30 minutes depending upon the size of the piece and the temperature and formulation of the bath. Then the blackened piece is removed from the bath, rinsed in cold water and either dried or, if desired, dipped in an oil or wax solution to impregnate or coat the black deposit with oil or wax for purposes of additional protection against oxidation and corrosion.

The following examples are given by way of illustration only and should not be considered limitative. The following table lists the ingredients of the present compositions and their effective amounts in parts by weight:

TABLE Petro AA (low chloride content naphthalene sodium sulfonate) 0.008

Example 2 Ingredients: Parts Sodium hydroxide 75 Sodium nitrate Sodium sulfide 10 Potassium dichromate 0.25

Example 3 Ingredients: Parts Sodium hydroxide 65 Sodium nitrate 2-mercaptoimidazoline 1'5 Sodium molybdate 0.25

The present blackening compositions, such as those set out in the above examples, are water-soluble solids which are stable in dry form for extended periods of time. In order to convert them to usable form, the compositions are dissolved in water in amounts ranging from about 2 to about 12 pounds per gallon of water and preferably from about 4 to 6 pounds per gallon of water depending upon the desired strength and effective life of the bath being prepared. Generally an immersion time of from 10 to 15 minutes in the bath is sufficient to form a dark black deposit which is smooth and even and which is firmly anchored to the stainless steel surface. The piece is removed from the composition and washed or rinsed. In cases where the bath contains a wetting agent it has been found that the blackened piece has a very small dragout or carries very little of the blackening composition from the bath upon its removal therefrom. This appears to be due to the lowering of the surface tension of the bath caused by the wetting agent so that the blackened pieces are thoroughly wetted by the bath and the residue is permitted to freely drain from the pieces upon their removal from the bath. This is advantageous for many reasons. Primarily it tends to increase the useful life of the blackening bath by preserving the ingredients therein. It also reduces the contamination of the rinsing bath to a substantial degree, substantially reduces the chances of improperly rinsed pieces becoming stained by residual salt and substantially improves the penetrating power of the blackening bath whereby intricately-shaped pieces can be blackened uniformly, even in areas inaccessible to conventional blackening baths.

In cases where it is desired to further treat the stainless steel to provide it with an additional oxidation-resisting, corrosion-resistant coating, the blackened pieces are immersed in any conventional rust-preventive wax, oil or resin composition, preferably water-displacing or polar types, to deposit in and/or over the blackened surface coating a protective coating or shield. This is not practical with pickled stainless steel which is not blackened since the surface has no affinity for an oil, wax or resin coating and attempts to apply and retain such coatings thereto are unsuccessful. However, the present blackened stainless steel surfaces are receptive to such protective coatings, either because of absorption or adsorption or both.

As protective coatings it is preferred to use thin solutions of oils and waxes which penetrate the blackened deposit and displace any water remaining therein as a residue of the aqueous blackening composition. When the pieces are withdrawn from the protective coating solutions the coatings dry quite rapidly in the form of very thin protective films or shields.

The phrase blackening composition as used herein and in the appended claims is intended to include compositions useful for applying jet black or off-black coatings including those in which the deposit is rather light or thin and which appear to have a gray color due partly to the show-through of the underlying stainless steel.

While the compositions of the present invention have been discussed primarily in connection with their use in the blackening of stainless steel, it should be understood that the present compositions are also useful for the blackening of other ferrous metals such as cast and malleable iron. This is important in view of the fact that it is difiicult to blacken cast iron and conventional blackening compositions deposit a reddish rather than a black coating thereon. However the present compositions are particularly adapted for the coloration of stainless steel in view of the many problems encountered in connection with the coloration of stainless steel and the difficulty of providing stainless steel with a coating receptive to protective outer coatings of oil or wax.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. The method of blackening stainless steel which comprises immersing the stainless steel for a period of from 1 to 30 minutes 111 an aqueous blackening bath to about 12 pounds soluble sulfur-bearing compound which is reactive with the stainless steel to form a black sulfur reaction product thereon and at least one water-soluble salt selected from the group consisting of from 0.0001 to 20 parts by weight of a molybdate salt and from 0.0001 to 2 parts by weight of a clichromate salt.

2. The method according to claim 1 in which the blackening bath also contains from 0.0001 to 15 parts by weight, based upon the weight of the blackening comagent which has a resistance to high alkalinity and oxidation at temperatures between 220 F. and 310 F.

3. The method according to claim 1 in which the blackening composition contains both a water-soluble molybdate salt and a water-soluble dichromate salt.

4. The method according to claim 1 in which the sulfur-bearing compound is 5. The method according molybdate salt is an alkali metal molybdate.

6. The method according to claim 1 in which the dichromate salt is an alkali metal dichromate.

7. The method according black sulfur reaction product parts by welght parts by weight of a water-soluble sulfur-bearing compound which is to 2 parts by weight of 9. An aqueous composition according to claim 8 which also contains from 0.0001 to 15 pa rts by weight, based upon the weight of the blackening composition, of a wetting agent which has a high resista nce to high alkalinity and oxidation at temperatures between 220 F. and

10. A composition according to claim 8 which includes both a water-soluble molybdate salt dichromate salt.

and a water-soluble 11. A composition according to claim 8 in which the sulfur-bearing compound is an alkali 12. A composition according to claim 8 in which the molybdate salt is an alkali metal molybclate.

13. A composition according to claim 18 in which the dichromate salt is an alkali metal dichromate.

14. A composition according to claim 8 in which the black sulfur reaction product is a black metal sulfide.

References Cited by the Examiner ALEXANDER H. BRODMERKEL SAMUEL H. BLECH, Examiner. J. B. EVANS, Assistant Examiner.

, Primary Examiner. 

1. THE METHOD OF BLACKENING STAINLESS STEEL WHICH COMPRISES IMMERSING THE STAINLESS STEEL FOR A PERIOD OF FROM 1 TO 30 MINUTES IN AN AQUEOUS BLACKENING BATH HAVING A TEMPERATURE BETWEEN 220*F. AND 310*F. AND CONSISTING ESSENTIALLY OF FROM ABOUT 2 TO ABOUT 12 POUNDS PER GALLON OF A BLACKENING COMPOSITION CONSISTING ESSENTIALLY OF FROM 40 TO 90 PARTS BY WEIGHT OF AN ALKALI METAL HYDROXIDE, FROM 0.1 TO 50 PARTS BY WEIGHT OF AN ALKALI METAL NITRATE, FROM 0.1 TO 50 PARTS BY WEIGHT OF A WATERSOLUBLE SULFUR-BEARING COMPOUND WHICH IS REACTIVE WITH THE STAINLESS STEEL TO FORM A BLACK SULFUR REACTION PRODUCT THEREON AND AT LEAST ONE WATER-SOLUBLE SALT SELECTED FROM THE GROUP CONSISTING OF FROM 0.0001 TO 20 PARTS BY WEIGHT OF A MOLYBDATE SALT AND FROM 0.0001 TO 2 PARTS BY WEIGHT OF A DICHROMATE SALT.
 8. AN AQUEOUS COMPOSITION FOR THE BLACKENING OF STAINLESS STEEL WHICH CONSISTS ESSENTIALLY OF FROM ABOUT 2 TO ABOUT 12 POUNDS PER GALLON OF A BLACKENING COMPOSITION CONSISTING ESSENTIALLY OF FROM 40 TO 90 PARTS BY WEIGHT OF AN ALKALI METAL HYDROXIDE, FROM 0.1 TO 50 PARTS BY WEIGHT OF AN ALKALI METAL NITRATE, FROM 0.1 TO 50 PARTS BY WEIGHT OF A WATER-SOLUBLE SULFUR-BEARING COMPOUND WHICH IS REACTIVE WITH THE STAINLESS STEEL TO FORM A BLACK SULFUR REACTION PRODUCT THEREON AND AT LEAST ONE WATER-SOLUBLE SALT SELECTED FROM THE GROUP CONSISTING OF FROM 0.0001 TO 20 PARTS BY WEIGHT OF A MOLYBDATE SALT AND FROM 0.0001 TO 2 PARTS BY WEIGHT OF A DICHROMATE SALT. 